JP2009-222418A (hereinafter referred to as Patent reference 1) discloses a known surface profile inspection device which irradiates a sheet of light generated by a laser light sheet source unit to an object to be measured, captures an image of the object to be measured by an image capturing unit, detects a light section line of the sheet of light from the image data captured by the image capturing unit, and attains the measured data of a three-dimensional profile configuration by the triangulation. According to the construction of Patent reference 1, an inspection result is determined by comparing measured data of the three-dimensional profile configuration and configuration dimension data.
The device disclosed in Patent reference 1 includes an optical system which includes a cylindrical lens for making a sheet of light irradiated from a laser light sheet projector to propagate (spread) in a sector shape in parallel to an optical axis. The optical system produces the sheet of light from high order coherent light and produces a light section line on a surface of the object to be measured by irradiating the sheet of light onto the object to be measured. Then, the optical system extracts (obtains) the light section line in the image data obtained by the image capturing means by processing the image and attains coordinates of the light section line by the principles of triangulation to obtain the surface profile of the object to be measured.
JP2009-198637A (hereinafter referred to as Patent reference 2) discloses a device which includes a poralization modulation portion for reducing a speckle noise accompanied with the irradiation of the coherent light, for example, a laser beam.
According to the construction of the known device, the narrower (the thinner) a linewidth of the light section line of the laser light sheet (the sheet of light) is, that is, the closer an irradiation position of the laser light sheet on the object to be measured to a focal position of the laser beam is, the more detection precision is enhanced because a resolution is increased. However, in a case where the irradiation position of the laser light sheet is away from the focal position of the laser beam, optical coherence is likely to be caused at portions of the light section line and a noise deriving from the optical coherence is likely to be caused in luminance distribution in the image data, thus the extraction of the light section line is assumed to be difficult. Namely, a speckle noise by the optical coherence is likely to be caused at the light section line.
As described in Patent reference 2, the speckle noise appears as fringe patterns in which bright points and dark points are randomly distributed. In a case where the speckle noise is generated at the light section line, the speckle noise causes variations in luminance of the light section line, which brings a drawback that the position of the light section line is assumed to be unclear and the light section line may not be measured. That is, when the speckle noise due to the optical coherence is generated at the light section line according to the known surface profile inspection device, the detection precision of the surface profile of the object to be measured may decline.
A need thus exists for a surface profile inspection device which is not susceptible to the drawback mentioned above.